Amino acids metabolism

Dietary nutrients are mainly absorbed in the small intestine. The digestive processes lead to the presence of amino acids in the lumen, in a free form or as small peptides. Both forms are absorbed by the intestinal cells through different transporters. The majority of peptides are then hydrolysed by intracellular peptidases resulting in the release of free amino acids in the hepatic portal blood.

Being taken up directly into the intracellular pool of a tissue, amino acids cannot be stored. They must follow either the anabolism route (protein synthesis) or go into the catabolic pathway.

Body proteins synthesis

Amino acids are important nutrients because they are the building blocks for proteins that have structural or functional functions in the organism. They can also be converted into other types of molecules with metabolic activities: e.g. tryptophan is a precursor of serotonin, a neuromediator involved in appetite, mood and behavior regulations.

All proteins are built from the ubiquitous set of the 20 amino acids encoded directly by the genetic code (Table 1): for each amino acid corresponds a specific code which is made up of a triplet of nucleotides named “codon” (Figure 2). The absence or poor availability of one amino acid is enough to slow down or stop the synthesis of protein.

Amino acids are brought by the transfer RNA (tRNA) into the ribosome that makes tRNA correspond to the genetic code presented by the messenger RNA. The ribosome then catalyzes the linkage between amino acids to form peptides and proteins.

Figure 2. Protein (peptide) synthesis

For instance, as lysine is one of the least available amino acid in vegetable proteins, it is described as the first limiting amino acid for pig growth performance and its correct supplementation is strategic for the growth of pigs. An increase in dietary lysine supply until its requirement, with a balanced amino acids profile, will result in an improvement in performance. Once the lysine supply is optimised, the next limiting amino acid is the one that limits protein synthesis the most. This depends mainly on the concentration of amino acids in feeds. For instance, in European diets, tryptophan is often limiting due to the low level of this amino acid in cereals. If tryptophan is not supplied at an adequate level, protein synthesis will slow down and the organism will not be able to use the other amino acids. The relative excess of these other amino acids will be catabolised. Avoiding dietary amino acids deficiencies increases the efficacy with which the animal will use the whole feed.

The feed-use amino acids are therefore used to compensate these deficiencies and facilitate the reduction of the dietary CP level by allowing a balanced profile of the indispensable amino acids.

These observations are the fundamentals of the ideal amino acids profile concept where the optimum indispensable amino acids supply is described in terms of ratios to lysine. In this concept, any deficiency of one of the indispensable amino acids will compromise growth and/or health, and can be illustrated by the Liebig’s barrel (Figure 3). The shortest stave determines the capacity of the barrel, as the dietary level of the first limiting amino acid determines the capacity of the animal to fully express its genetic potential.

By supplementing the feed with the feed-use amino acid corresponding to the first limiting amino acid, the animal performance capacity is improved until the level allowed by the next limiting amino acid.

Figure 3. The Liebig’s barrel

Amino acid catabolism

The amino acids can not be stored by the organism and in case they can not be used by the organism (e.g. excesses), they will be catabolised. This catabolism is mainly a separation of nitrogen from the carbon skeleton. The carbon skeleton will be used to synthesise glucose and/or fatty acids. The nitrogen can be recycled by the organism (protein “turn-over”) or excreted under the form of urea (or uric acid for poultry) and NH3 in the manure. An excessive NH3 excretion contribute to the nitrogen pollution from livestock. The catabolism takes place in the liver, but for some specific amino acids, the branched-chain amino acids (BCAA: valine, isoleucine and leucine), this catabolism is initiated in the muscle and the second step takes place in the liver (Figure 4).

Figure 4. Global pattern of the branched chain amino acids metabolism

From a practical aspect, supply of amino acids over their requirements and unbalanced supply between amino acids must be avoided to fully benefit of the feed efficiency. Working with low CP diets and feed-use amino acids allows achieving a balanced supply of amino acids without wasting dietary energy for catabolism.